That is going to be a tough one. Try Jack First Gun Parts. Call them and see what they have.. Or Western Gunparts in Canada.
If I ID it right as a 1910 or 1934 Mauser .32, those, if I recall correctly, have a leaf spring that retains the magazine. It's in the shape of an elongated cross and the base has a folded knob you pull back to release the mag. Not a hard part to make if you have the original for a template. Which is good because it's a terrible design.